1. Field of the Invention
The present invention relates to semiconductor device mounting structures that are resistant to impact and also show ease of repair, their manufacturing methods, and removal methods of the semiconductor device.
2. Background Art
Electronic devices are becoming ever smaller and more sophisticated, and the use of small portable mobile information devices, including mobile phones and personal computers, is increasingly widespread. Electronic components mounted on circuit boards in these devices contain numerous semiconductor devices. Semiconductor devices are normally mounted in the form of a package, in which a semiconductor chip is mounted on an interposer and molded with resin. More specifically, ball grid arrays (hereinafter abbreviated to “BGAs”) and chip scale packages (“CSPs”) are the commonest types of package used for semiconductor devices. BGAs have a structure in which connecting terminals, typically formed of ball-shaped solder bumps, are aligned in an array on one main face of the semiconductor device facing a circuit board.
Since mobile information devices are portable, they require strong resistance to impact if dropped. A semiconductor device, in which functional circuits including logic circuits and memory circuits are densely packed, has a vast number of connecting terminals. As a result, BGA and CSP packages have very small terminals and fine terminal pitch. In addition, semiconductor devices formed of packages such as BGAs and CSPs have connecting terminals aligned on a grid, giving a large mounting area. Stress may thus be concentrated on the connecting terminals disposed near the edge of the semiconductor device package due to deformation of the circuit board or stress caused by dropping. As a result, soldering between the semiconductor device and circuit board may be damaged or broken.
This has prompted studies of a means of improving the reliability of connection between the semiconductor device and the circuit board against drop impact. One general approach is first to solder the semiconductor device and circuit board, and then reinforce the connection by fixing the semiconductor device to the circuit board using a thermosetting adhesive such as epoxy resin. This reduces concentration of stress, caused typically by drop impact, on the connecting terminals, thus improving impact resistance. However, if a characteristic failure is found in the semiconductor device after being mounted on the board, the semiconductor device needs to be replaced. In this case, the thermosetting adhesive on the circuit board needs to be removed to allow re-connection of a replacement semiconductor device to the circuit board. However, tight adhesion of thermosetting adhesive on the circuit board hinders its removal, making repair difficult. As a result, there is no choice but to dispose of the entire circuit board if a characteristic failure is found.
To solve this issue, Japanese Patent Unexamined Publication No. 2006-100457 proposes a mounting structure that has high impact resistance and also enables repair and reworking. More specifically, it proposes a semiconductor device mounting structure that employs resin consisting of two layers for bonding the semiconductor device to the circuit board. One layer uses resin that can be removed, and the other layer uses resin with high mechanical strength. The semiconductor can thus be removed from the circuit board by heating the removable resin layer until it becomes fluid, to allow removal when repair is needed, such as when replacing the semiconductor device.
Japanese Patent Unexamined Publication No. 2005-332970 proposes a semiconductor device mounting structure in which resin composite is injected between the semiconductor device and the circuit board and then cured. This resin composite is made of organic thermally expandable particles encapsulating an organic solution and thermosetting adhesive resin. In this semiconductor device mounting structure, which has strong impact resistance as configured above, the organic solution in the organic thermally expandable particles boils and evaporates by heating, causing rapid volume expansion. This changes the resin hardener that bonds the semiconductor device to the circuit board into an extremely brittle and porous structure. The semiconductor device thus becomes removable, allowing the semiconductor device mounting structure to be repaired.
Japanese Patent Unexamined Publication No. 2001-107019 also proposes the inclusion of a foamable material in the adhesive used to firmly bond the semiconductor device to the circuit board so that the semiconductor device can be removed from the circuit board by making the foamable material foam significantly at the time of repair. In other words, when adhesive including paste and foamable material is heated or microwaved, the foamable material is foamed. This generates numerous pores in the paste, and makes the paste brittle, facilitating removal of the semiconductor device from the circuit board.
However, the prior arts disclosed in the above Japanese Patent Unexamined Publication Nos. 2005-332970 and 2001-107019 require the heating temperature to be very precisely controlled to be able to effectively expand the thermally expandable particles or foamable material to permit removal of the semiconductor device from the circuit board. A temperature range that generates effective expansion force in the thermally expandable particles or foamable material is typically 10 to 20° C. This temperature range is not sufficient for application to mass-production processes.
In other words, if the temperature needed to attain maximum expansion force is below the melting point of the solder bumps, the thermally expandable particles or foamable material cannot gain sufficient expansion force. As a result, not all the thermosetting adhesive can be made sufficiently brittle, preventing easy removal of the semiconductor device from the circuit board. If the amount of thermally expandable particles or foamable material contained in adhesive is increased so as to increase the expansion force, the semiconductor device will be lifted from the circuit board with the solder bumps unmelted. This may cause tearing damage due to stress applied to the copper foil on the circuit board. As a result, re-connection of a new semiconductor device onto an electrode of the circuit board becomes impossible, necessitating disposal of the circuit board.
On the other hand, if the temperature for attaining the maximum expansion force is the same or higher than the melting point of the solder bumps, the circuit board will not be damaged during removal of the semiconductor device, since the solder bumps are sufficiently molten, even if the thermally expandable particles or foamable material expands sufficiently at this temperature. However, the thermally expandable particles or foamable material included in the adhesive need to provide sufficient expansion force to facilitate removal of the semiconductor device by making all of the adhesive fully brittle. For this purpose, the solder bumps need to be retained at a temperature higher than their melting point for a long period of time. As a result, the components mounted around the semiconductor device on the circuit board may become thermally damaged.
In the prior art disclosed in Japanese Patent Unexamined Publication No. 2006-100457, it is difficult to remove the semiconductor device from the circuit board in the semiconductor device mounting structure without leaving any residue on the circuit board, and then to re-connect a new semiconductor device in that state after removal.